PhD Oral Exam - Qi Feng, Civil Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

The presence of plastic fragments in the environment is a growing global concern. The coast is particularly vulnerable to microplastic (MP) pollution. However, previous lab studies regarding the transport of MPs in porous media mainly focused on MP properties, substrate properties, and porewater chemistry, and less focused on complicated environmental processes. Additionally, field experiments are less capable of disentangling the complex interplay of various factors. Therefore, this study aims to investigate the transport of MPs in porous media combined with typical coastal processes. Firstly, a weathering experiment was conducted to comprehensively reveal the changes in MPs in the environment. The results indicated that seawater aging mainly affected the physical properties of MPs, increasing its surface pores and hydrophilicity. Ultraviolent (UV) aging significantly affected both the physical and chemical properties of MPs, which increased its hydrophilicity and crystallinity and introduced oxygen-containing functional groups onto MPs. The two-dimensional correlation spectroscopy (2D COS) analysis confirmed the evolution of oxygen-containing functional groups from C–O to C=O.

Then, the detachment of MPs from porous media under various water content conditions combined with flow patterns was studied. For both the wet and dry conditions, the increase in flow rates decreased the detachment of hydrophobic polyethylene (PE) of two sizes and hydrophilic polymethylmethacrylate (PMMA). Intermittent flow led to flow peak and more MP detachment compared to steady flow. A stepwise decrease in the ionic strength resulted in abrupt peaks followed by prolonged tails with the peak timing coinciding with the arrival of the ionic strength front at the column outlet. Each stepwise increase in flow rate led to a sharp peak followed by slow release over several pore volumes. However, drying significantly impeded the detachment of MPs compared to wet conditions irrespective of the flow patterns. Furthermore, the release of MPs decreased pronouncedly with the increase in air drying time from 0 to 72 hr. Hydrus-1D two-site kinetic models were used to simulate time-dependent processes and demonstrate good fitting.

Tide is a typical coastal process that has profound influence on many biological and abiotic processes. In the following study, the effects of tidal cycles on transport of MPs (4-6 μm PE1; 125 μm PE2; and 5-6 μm polytetrafluoroethylene, PFTE) in porous media combined with various environmental and MPs properties were systemically investigated. The results indicated that smaller substrate sizes exhibited higher retention percentages compared to those of larger substrate sizes under different tidal cycles. In terms of the size of MPs, a larger size (same density) was found to result in enhanced retention of MPs in the column. As the number of tidal cycles increased, although the transport of MPs from the substrate to the water phase was enhanced, PE1 was washed out more with the change in water level, compared to PTFE. Additionally, more MPs were retained in the column with the increase of salinity and the decrease of flow velocity under the same tidal cycles. Ultraviolet and seawater aged PE1 showed enhanced transport, while aged PTFE showed enhanced retention under the same tidal cycles.

To expand our understanding of MP mobilization by tidal forces, the influence of dynamic fluctuations of capillary fringe on the transport of MPs was explored. Both experimental and Hydrus-2D modeling results confirmed that increasing cycles of water table fluctuation led to the rise of capillary fringe. An increase in the cycles of water table fluctuations didn’t significantly change the overall MPs retention percentages in 0.5 mm substrate but altered the MPs distribution along the column. In 1 mm and 2 mm substrate, the increase of cycle numbers enhanced the MPs transport from substrate to the water below. In terms of the size of the MPs, more 20−25 μm PE2 were retained in substrate compared to 4−6 μm PE1 under the same number of fluctuation cycles. High-density 5−6 μm PFTE exhibited higher retention percentages compared to PE1 particles. UV aging for 60 days enhanced PE1 transport along the column, while 60 days of seawater aging did not affect PE1 transport greatly. For PTFE, ultraviolet and seawater aging enhanced its retention in substrate. The retention percentages of both PE1 and PTFE in column increased with the elevated ionic strength and the decrease of fluctuation velocity. The results highlight that capillary fringe fluctuation can serve as a pathway to relocate MPs to the tidal aquifer.

Finally, a mesoscale tank experiment was conducted to simulate the infiltration and resuspension of MPs in a slope substrate under the influence of repeated tidal forces. The results imply that large, high-density, and less flatty particles tend to be distributed in the lower tidal zone and deeper substrate layers. The obtained observation contributes valuable insights into the behavior, transport, and redistribution of MPs in complex environmental systems. The findings enhance our understanding of MP fate and distribution, assisting in the development of strategies for mitigating MP pollution and managing its impact in coastal areas.